Recently I bought a Samsung EVO 840 120GB SSD and a DeLock ESATA/USB3.0 caddy. My work laptop ( Dell Precision M6400 ) has an eSATAP socket, which looks like an ordinary USB socket but with a slightly wider indent at the top. Once I switched the BIOS mode from SATA to AHCI I was able to boot off the SSD connected through the eSATAP port. Having formatted the disk in Windows 7 for NTFS, I resized the partition in Windows and selected the unused space for the primary partition and swap space in the Ubuntu installer. Although it is slightly clumsy when moving the laptop, it is a great way to get a fast Linux experience without changing your internal disk.
Performance is noticeably better when compared to the 5400rpm internal disk ( 10 seconds from GRUB to Login screen ) and Ubuntu 12.04 has better support for the USB 3.0 ExpressCard I am using. I also tested using a virtual machine with the VDI located on the SSD while running Windows 7 on the 5400rpm disk as the host, the VM ran noticeably faster. I am impressed with the stability of eSATA, I have experienced a little hanging but I do not believe it is an issue with the eSATA connection.
It is possible to get a caddy to replace your optical drive, allowing you to have a second hard disk.